What is SPL in Audio?
With seemingly endless gear specifications to consider, it’s no wonder many of us new to audio production are slightly confused. There’s a reason they call the really smart guys in the control room “engineers.” This stuff can be extremely technical! But fortunately for you, there’s plenty of information out there on the internet in this day and age to clear things up, if you’re willing to learn. In this technical article, we’ll answer the question you came here for: What is SPL in audio? Without further ado, let’s get to it.
Sound Pressure Level
SPL stands for “sound pressure level.” As we already know from this article, sound is simply pressure oscillations in the air. Sound is basically an extremely (thousands of times per second) rapid compression and expansion of the air molecules. These pressure waves cause little hairs in your ears to vibrate. The more intense the pressure waves are, the louder the sound source will sound.
But what is SPL in audio, exactly? So since sound is basically just pressure waves, sound pressure level is a measurement of the pressure waves. SPL is measured in a unit called decibels, abbreviated dB. In audio gear and microphone specifications, you’ll commonly see dB SPL. Let’s talk a little more about decibels.
Decibels & dB SPL
A decibel is a unit of measure that utilizes the logarithmic scale. What’s that mean? Well, an increase in 10 dB means the sound intensity is multiplied by 10 to the +1 power. For example, 20 dB is 10 times louder than 10 dB, and 70 dB is 1,000,000 times louder than 10 dB. Every addition or subtraction of 10 dB adds or subtracts an extra zero to the multiplier. Decibels are used everywhere in the scientific community, not just for measuring sound.
Sound pressure level is measured relative to the auditory threshold. This means that 0 dB SPL is the threshold of hearing. Anything below 0 dB SPL is inaudible. An example of places with negative dB SPL are anechoic chambers. These special rooms are designed to absorb all sound and acoustic reflections, and they’re used to test everything from headphones to fighter jets. Here are some examples of common sound sources and their sound pressure levels:
Keep in mind that SPL is also dependent on the distance from the source. For instance, a jet engine at 3 ft produces a vastly higher SPL than one at 300 ft.
Microphones, Speakers, & Max SPL
A microphone is a electroacoustical device that converts the sound pressure in the air into an electrical signal. Microphones are used for both measuring SPL and, as we all know, recording and transmitting audio. Microphones vary in their operation. For instance, condenser microphones pick up on a change in capacitance, while ribbon microphones measure the velocity of the internal components.
A speaker is also an electroacoustical device, but it converts electricity to sound. A speaker is basically a microphone operating in reverse.
A common specification you’ll see on microphone, speaker, and other audio gear data sheets is max SPL. This is simply the maximum sound pressure level in dB that the microphone or speaker can handle without producing an unacceptable level of distortion. “Unacceptable” is defined as greater than 0.5% THD. Mics with high max SPL ratings are great for recording percussion and other very loud sources in general. Speakers with high max SPLs are ideal for large venues where long distance coverage is needed.
How much max SPL is necessary?
Well, looking at our table, we can get an idea of what the max SPL spec should be if we’re looking to buy a microphone. Unless you’re recording audio from stun grenades and jet engines in your bedroom (bad idea, trust me) you’re not going to really need anything with an exceptionally high max SPL.
The exception here is drums. Kick drums can exceed 140 dB SPL, but according to Shure, have never reached above 150 (which is 10 times louder than 140). A trumpet can produce 155 dB SPL at very close distances, theoretically. Tambourines are also surprisingly loud.
Back to our question – if you’re recording drums, a dynamic mic may be the better choice since they don’t distort nearly as easily as condensers. But if you do use a condenser, shoot for at least 140 max SPL, just to be safe. Most modern condensers can handle anything else you throw at them.